1. Field of the Invention
The present invention concerns a method to determine an MR system-dependent phase information of a phase value in an MR phase image data set, and an MR system with which the MR system-dependent phase information can be calculated.
2. Description of the Prior Art
Phase imaging has a broad field of application in magnetic resonance tomography (MRT). For example, the phase information contained in the MR signal can be used in susceptibility-weighted MRT, phase contrast MRT, for flow measurement, to determine the fat content in a tissue, and for thermometry. In thermometry or thermotherapy methods, the temperature in tumor cells is specifically increased in order to kill the cells or to make the tumor cells sensitive to accompanying therapy measures such as chemotherapy or radiation therapy. For example, tissue heating can take place via lasers or ultrasound, or RF antennas. In order to not significantly stress the healthy tissue due to the increased temperature, a temperature monitoring of the heated tissue is required. Due to the temperature dependency of some MR parameters such as the chemical shift, MRT has the potential to non-invasively show temperature changes. Given temperature monitoring based on the temperature dependency of the chemical shift, the resonance frequency altered by the temperature increase is detected in a modified phase position in the image point. However, since the temperature dependency is slight, with −0.01 ppm/° C., it is important to know the phase information dependent on the MR system or, respectively, the phase change dependent on the MR system so that the temperature information determined from the phase information is not adulterated. Given temperature imaging based on the chemical shift, only temperature changes can be shown via calculations of phase changes. It is also important to separate the system-induced phase change from the temperature-induced phase change. The phase drift existing over time due to the change of the B0 field can be determined via reference image points that are not heated, for example. Given this difference calculation of phase data sets acquired at different points in time, it can occur that the patient moves or the magnetic environment changes between the measurements.
One problem given the use of phase information is the presence of water and fat molecules in a tissue. First, the resonance frequency of fat and water differ by approximately 3.5 ppm, and second, the chemical shift of fat is not temperature-dependent. In a method named for Dixon, the phase drift generated due to B0 field inhomogeneities is estimated in order to separate fat from water. The standard assumption in Dixon fat-water separation is that the signal in a voxel is generated by two different populations: fat molecules and water molecules. If the fat proportion is designated as r, the water proportion is (1−r), wherein r is between 0 and 1. Each molecule has its own angular frequency ωfat for fat and ωwater for water. Due to the different resonance frequency, the signal proportions of the two molecules are either in phase or counter-phase depending on the echo time.
Furthermore, techniques known as reference-less methods for generation of phase image data sets are known in which the system-dependent phase information is not measured in an additional reference subject.
For example, in methods with reference subjects the reference subject can be the patient himself, wherein a reference data set is measured before heating. In flow measurement a flow-coding gradient can be activated and deactivated. Flow measurements are often measured without reference subject. Given these reference-less approaches it is assumed that the MR system-dependent phase information changes only slowly over the field of view of the image, such that—if a reliable phase information is known at some image points—the MR system-dependent phase change can be approximated via interpolation, for example via a low order polynomial. However, the selection of valid phase reference points is necessary for this, based on which the phase development across the image can be approximated. It is thus necessary to differentiate between valid image points and other image points in which the MR system-dependent phase influence must be estimated.
This division into valid and invalid image points is difficult to implement since image points must be found in which the tissue is not heated, nor does a possible fat-water combination adulterate the phase information.